Makes sense. Mono was necessary in the “old .NET” world, where runtimes were ties to Windows versions and the framework was a pure Windows framework. Mono made it possible to run old dotNET framework versions (up to 4.8) on other OSes.
Since dotNET Core and then dotNET 5 and higher, the framework itself is cross-platform so Mono is mot necessary anymore, except for backwards compatibility for apps that use a now unsupported framework.
So it makes sense that Microsoft, after dropping the old dotNET Framework versions, also wants to stop supporting the cross-platform library that was only needed by those old versions.